However, I'm trying to create custom extensions (e.g. file.customname, instead of file.aspx), similar to how Apache/PHP/htaccess can handle this. (I think it's mod_rewrite or .htaccess, forgotten which now!)

What would I do to get these custom file types (all serving .NET files) working?

The extensions are given custom names by me, simply for testing purposes

This is a tourism site [a testing one, btw], and it's working OK on localhost for basic pages, but I wanted to know how to do this.

How would I go about this? I've had a look on Google etc. but custom HTTP handlers was all I could really find!